1. Field of the invention
The invention relates to extrusion technology for thermioplastically deformable plastics, especially a process and a device for stable regulation of the bulb angle of plastic sheets for foils or plates.
2. Description of the Related Art
Extrusion facilities for heat-sensitive plastics are sufficiently known. Generally, roller calenders are used for bilateral smoothing of the surfaces of plastic sheets for plates or foils. The rollers are intended to cool and shape the melted plastic. The rollers take up the plastic vertically emerging in a molten state from a fishtail die into the calender slit between two driven intake rollers. Generally, the plastic sheet is taken off by one of the intake rollers and diverted horizontally with the aid of another roller which presses upon the intake roller. The plastic sheet acquires a defined thickness and a smooth surface by being pressed through the calender opening.
Factors which influence the surface characteristics of plastic sheets for bilaterally smoothed plates and foils are the composition of the polymer, the temperature and viscosity of the polymer melt stream, the temperature and surface characteristics of the rollers, the size of the calender slit and the speed of the rollers.
A homogenous passage of the plastic through the calender opening is extremely important for flawless surface properties of the extruded material. Great significance attaches to the height of so-called "bulb" which forms in the calender slit between the intake rollers due to the back pressure of the polymer melt. Fluctuations due to local mass flow differences in the polymer melt leaving the extrusion nozzle lead to different melt accelerations on the surfaces of the intake rollers, owing to which the height of the bulb also changes.
The height of the bulb should only fluctuate within the smallest tolerances possible. If it is too low, contact of the extrudate on one of the intake rollers can be lost. This leads to a poor extrudate surface as a consequence and can also lead to sticking on the other intake roller due to the sharply altered cooling conditions. This so-called winding then leads to the breakdown of the process and generally to a standstill for several hours.
An excessively high bulb leads to disturbances of the extrudate surface owing to so-called bulb rolling. In this connection, the bulb is drawn in loop-like and, in a certain way, rolled up from above by contact with the entering extrudate stream. This leads to the formation of streaks running crosswise to the extrudate surface. In an extreme case, the stagnation bulb also reaches the nozzle lip and contaminates it, which again leads to disturbance of the extrudate surface. Cleaning the nozzle can only take place in connection with a facility shutdown.
Several procedures for controlling the bulb height are known. It is usual in practice for a manual readjustment of the bulb height to take place by altering the roller speed in conjunction with measuring the thickness of the cooled plastic sheet. This process has the disadvantage that since the intervention is undertaken manually, it is consequently open to subjective influences. Due to the relatively late measurement of thickness, a relatively high amount of wastage occurs until the adjustments lie within the tolerance sought. Furthermore, direct measurements of the height to the bulb through mechanical sensors or according to the laser triangulation method are known. The mechanical method is rather inexact, while the optical method has the disadvantage that it is prone to error owing to the high temperatures in the region of the stagnation bulb and the density fluctuation of the air caused thereby.
An automatic control operating with the aid of an adjusting wheel is known from DE 40 33 661 C2. The bearing pressure is determined on one of the intake rollers as a measure for the thickness of the sheet of material and is regulated by roller speed. Since the bearing pressure, however, is not only dependent upon the thickness of the sheet of material, but also upon the viscosity and temperature of the plastic stream, this control is also unsatisfactory.
Widthwise control of the bulb angle for the extrusion of transparent foils with a thickness under 5 mm is known from G 92 12 406.2. Here, measurement of the orientation double refraction angle of the extruded foil is adduced as a measure for the bulb height and regulated by changing the adjustment of the mass flow of the extrusion nozzle.
GR patent 85.1420 describes a device for the widthwise control of the bulb angle in the production of a bilaterally smoothed plastic sheet. The bulb angle is held constant since the surface temperature of the sheet after leaving the roller gap is used as an input. The molding mass discharge is increased or diminished upon any deviation from the expected temperature at a the constant operating state in continuous operation. The procedure has the disadvantage that it is too imprecise. In EP-A 0 429 161, a largely identical device is described as in GR patent 85.1420.
G 92 08 837.6 describes a device for eliminating the influence of roller impact on thickness distribution in an extruded, surface-smoothed foil. There, an even bulb angle is created on the calender roller by means of a traversing bulb measurement and/or through a traversing foil thickness measurement. Production-related non-circularity is cited as a disturbing factor which causes sinusoidal fluctuations in thickness in the extruded and smoothed foil sheet. If measured thickness values which reproduce these sinusoidal fluctuations in thickness are used for an automatic control of the extrusion nozzle, this can bring about permanent fluctuations in bulb height which for their part produce turbulences in the bulb. As a result, the bulb rollers can lead to optical irregularities in the foil. The sinusoidal foil thickness fluctuations are eliminated through a traversing thickness measurement on the foil sheet with a sensing element, for example, a capacitive position pick-up or an interferometer which conducts measurements while being displaced along the direction of delivery of the foil sheet around half the circumference of the calender roller, and by an averaging of recorded thickness values in the same position in the delivery direction. The second measurement in any given case is started by a digit emitter displaced half a roller rotation in relation to the first measurement. Fluctuations in thickness of the extruded foil can thus be kept within a maximum deviation of 5% on the average.
In Spang, A. and Wusternberg (1993).--"On-line Thickness Measurement: NonContact Process for Foil-Like Materials, " in: Plastics 83, 11, p. 894-897, various thickness measuring procedures on calendered foils are described. Clearly recognizable thickness fluctuations with a sinusoidal course are described as disturbances. The time of oscillation corresponds to the circumference of the calender roller in this connection.